This invention relates to degasification of coal seams, and more particularly to an improved method of treating a coal seam to increase the gas permeability thereof. Many coal seams contain methane and other undesirable gases which create a mining hazard. Several methods are currently in use to reduce the amount of undesirable gases in coal seams in advance of mining.
U.S. Pat. No. 4,043,395 describes a process for removing methane from a coal seam in which a carbon dioxide-containing fluid is introduced into the coal deposit and held therein for a period sufficient to enable a substantial amount of methane to be desorbed from the surfaces of the coal. Following the holding period, the injected fluid and desorbed methane are recovered from the coal seam.
U.S. Pat. No. 3,384,416 describes a method of fracturing and degassing of coal seams by injecting a volatile liquid into the coal seam to fracture it. The pressure is then released and the volatile material and coal seam gas are withdrawn from the fractured coal seam.
U.S. Pat. No. 1,867,758 describes an early technique for degasifying coal seams by forming a tunnel system through the seam and applying a partial vacuum to the tunnel system.
U.S. Pat. Nos. 3,650,564 and 3,934,649 describe drilling processes for degasifying coal seams. The processes described in these patents are not particularly pertinent to the process of the present invention, but they are mentioned to emphasize the extent of work that has been carried out to effect degasification of coal seams.
U.S. Pat. No. 4,044,833 describes a method of fracturing a subterranean formation using an acid foam as the fracturing fluid.
U.S. Pat. No. 4,126,181 describes an improved method of fracturing a subterranean formation using a foam.
U.S. Pat. No. 4,080,419 describes a process for leaching fragmented ore with a reagent-carrying foam.
It has been proposed to inject a coal solvent into a coal seam to partially dissolve the coal, particularly around the existing fractures in the coal, to improve gas permeability from the coal seam.
While numerous techniques have been utilized in the past for degasifying coal seams, these techniques, while successful to varying degrees, have all had shortcomings. Drilling of degasification boreholes through the coal seams is time consuming, expensive, and difficult. Fracturing of coal seams can be effective, but can weaken a mine roof structure with subsequent hazards to the mining operation. Injection of a coal solvent requires a great deal of expensive solvent, and the injected solvent is difficult to recover. Thus, there has been a continuing need for improvements.